The present invention relates to electrodeless light sources and, more particularly, to such sources which are excited by high frequency power, such as in the range of 100 MHz to 300 GHz.
There have been, historically, three basic methods of exciting discharges without electrodes. The first method uses the discharge as a lossy part of either the capacitance or inductance of a "tank" circuit. This method is used to advantage only at frequencies where the dimensions of the lamp are much smaller than the wavelength of excitation. Also, in this method, there are power losses due to radiation and shifts in frequency upon start-up. A second method of exciting electrodeless lamps with microwave power is to place the lamp in the path of radiation from a directional antenna. However, since free propagation of microwave power occurs, there is an inherent inefficiency and some of the power is scattered thereby endangering persons in the area.
A third method uses a resonant cavity which contains the lamp, a frequency tuning stub and a device for matching the lamp cavity impedance to that of the source and transmission line. Examples of devices according to this method may be found in "Microwave Discharge Cavities Operating at 2450 MHz" by F. C. Fehsenfeld et al., Review of Scientific Instruments, Volume 36, Number 3, (March, 1965). This publication describes several types of tunable cavities. In one type, cavity No. 5, the discharge cavity transfers power from the source to the lamp, and the resonant structure of the cavity increases the electric field in the gas of the lamp. The presence of a discharge in the resonator changes the resonant frequency and also changes the loaded Q factor. Therefore, it is necessary to provide both tuning (frequency) and matching (impedance) adjustments to obtain efficient operation over a wide range of discharge conditions. The tuning stud is first adjusted for a minimum reflected power with the minimum probe penetration. Next, the probe (impedance) is adjusted. Since these two operations are not independent, successive readjustments are required to achieve optimum efficiency.
All of these tunable cavities have features which make them less than ideally suited for use in an electrodeless light source. To make cavity type systems useful economically, the cavity must be small enough so that it would be feasible to use such systems in place of the conventional electrodecontaining lamp. Resonant cavities are too large and must be larger if lower microwave frequencies are used. One resonant cavity for 2450 MHz operation has 4 inches as its greatest dimension; the size would be even larger for operation at 915 MHz which is a standard microwave frequency for consumer use, such as with microwave ovens. Operation at this lower frequency is also advantageous from the view that the greater the frequency the more expensive the microwave power source becomes. The known tunable cavity has a less than optimum shape because the lamp is substantially enclosed by the resonant cavity housing, thereby impeding the transmission of light.